ext/uri: Increases test coverage

[NickSdot]

Edit: WIP -- needs work after #23551 and #23286!

Depends on #23286:

This is fully Codex made. I threw a coverage report at it to close uncovered lines/branch gaps. It came up with a tonne of edge cases; this PR is the result of me massaging it a bit, and after filtering out nonsense that did not actually increase coverage as well as things that seemed too esoteric. So everything here increases coverage even though things might seem already tested -- that's because of using slightly different api paths.

Coverage: ext/uri

Base: 98a259edd23cb132ac0b626dd9d4a8edacca4b3e upstream/master
Tree: 769f75be83474ca81d9bc3b236cd1e798990885e working tree

+--------+-------+---------+--------------------+-------------------+--------+----------+
|        | Tests | Sources |              Lines |          Branches |   Time |   Memory |
+--------+-------+---------+--------------------+-------------------+--------+----------+
| Base   |  4485 |       7 | 1710/1934 (88.42%) |  600/968 (61.98%) | 70.26s | 153.7 MB |
| Tree   |  4512 |       7 | 1777/1934 (91.88%) |  638/968 (65.91%) | 63.03s | 153.7 MB |
| Change |   +27 |       0 |  +67 / -0 (+3.46%) | +38 / -0 (+3.93%) | -7.23s |  +0.0 MB |